Improve Ada dynamic range type handling.

Improve Ada dynamic range type handling.

Consider the following declaration in Ada...

   type Array_Type is array (L .. U) of Natural;

... where L and U are parameters of the function where the declaration
above was made. At the moment, GDB relies on descriptive types in order
to properly decode the array bounds. For instance, if L was 5, and U
was 10, we would see the following:

    (gdb) ptype array_type
    type = array (5 .. 10) of natural
    (gdb) maintenance set ada ignore-descriptive-types
    (gdb) ptype array_type
    type = array (1 .. 28544912) of natural

This patch enhances ada_discrete_type_{high,low}_bound to resolve
any dynamicity.  This is sufficient to fix the case of the upper bound.
For the lower bound, the dwarf2read module does not handle dynamic
lower bounds yet, but once it does, the lower bound should be correctly
handled as well [1].

gdb/ChangeLog:

        * ada-lang.c (ada_discrete_type_high_bound): Resolve the type's
        dynamic bounds before computing its upper bound.
        (ada_discrete_type_low_bound): Same as above with the lower bound.

[1]: The reason why we do not enhance dwarf2read to handle dynamic
lower bounds ahead of this patch is because it unveils some latent
issues such as this one.